SCRs with checker board layouts

ABSTRACT

An Electro-Static Discharge (ESD) protection circuit includes a plurality of groups of p-type heavily doped semiconductor strips (p+ strips) and a plurality of groups of n-type heavily doped semiconductor strips (n+ strips) forming an array having a plurality of rows and columns. In each of the rows and the columns, the plurality of groups of p+ strips and the plurality of groups of n+ strips are allocated in an alternating layout. The ESD protection circuit further includes a plurality of gate stacks, each including a first edge aligned to an edge of a group in the plurality of groups of p+ strips, and a second edge aligned to an edge of a group in the plurality of groups of n+ strips.

BACKGROUND

Silicon-Controlled Rectifier (SCR) devices have excellent Electro-Static Discharge (ESD) immunity and are good candidates for low-capacitance applications, which include Radio Frequency (RF) and high speed devices. SCRs, however, have low turn-on speed and high trigger voltages, and hence are not suitable for operations that need high turn-on speed and low trigger voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a top view of a Silicon-Controlled Rectifier (SCR) and diode string unit (referred to as SCR/diode-string unit hereinafter) in accordance with some exemplary embodiments, wherein two diodes are connected serially to form the diode string;

FIG. 2 illustrates a cross-sectional view of the SCR/diode-string unit in accordance with some exemplary embodiments;

FIG. 3 illustrates a circuit diagram of the SCR/diode-string unit in FIG. 2;

FIG. 4 illustrates a top view of a plurality of SCR/diode-string units connected in parallel;

FIG. 5A illustrates a top view of an SCR/diode-string combo unit in accordance with alternative embodiments, wherein four diodes are connected in series to form a diode string;

FIG. 5B illustrates a top view of an SCR/diode-string combo unit in accordance with alternative embodiments, wherein three diodes are connected in series to form a diode string;

FIG. 6 illustrates a cross-sectional view of the SCR/diode-string combo unit in FIG. 5A;

FIG. 7 illustrates a circuit diagram of the SCR/diode-string combo unit in FIG. 5A;

FIG. 8A illustrates a top view of a diode-string-free SCR unit in accordance with some embodiments;

FIG. 8B illustrates a top view of a diode-string-free SCR unit in accordance with some embodiments, wherein the diode-string-free SCR unit includes a half of the diode-string-free SCR unit shown in FIG. 8A;

FIG. 9 illustrates an Electro-Static Discharge (ESD) device including a plurality of SCR/diode-string units connected in parallel;

FIG. 10 illustrates an ESD device including SCR/diode-string unit(s) and diode-string-free SCR units, wherein a ratio of the number of SCR/diode-string unit(s) to the number of diode-string-free SCR units is equal to 1:1;

FIG. 11 illustrates an ESD device including SCR/diode-string unit(s) and diode-string-free SCR units, wherein a ratio of the number of SCR/diode-string unit(s) to the number of diode-string-free SCR units is equal to 1:2;

FIG. 12 illustrates an ESD device including SCR/diode-string unit(s) and diode-string-free SCR units, wherein a ratio of the number of SCR/diode-string unit(s) to the number of diode-string-free SCR units is equal to 1:3; and

FIG. 13 illustrates a table showing the relationship between the holding voltages of SCR/diode-string units and the number of diodes in the diode string of the respective SCR/diode-string units.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative, and do not limit the scope of the disclosure.

Electro-Static Discharge (ESD) protection devices comprising Silicon-Controlled Rectifiers (SCR) and diode strings are provided in accordance with various exemplary embodiments. The variations of the ESD protection devices in accordance with some embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIG. 1 illustrates a top view of a unit (referred to as an SCR/diode-string unit hereinafter) that includes an SCR and a diode string therein. SCR/diode-string unit 20 includes a plurality of semiconductor strips 22, which are surrounded by Shallow Trench Isolation (STI) region(s) 24. Referring to FIG. 2, semiconductor strips 22 include portions 22′ between STI regions 24, and may, or may not, include semiconductor fins 22″ that are over the top surfaces of STI regions 24.

Referring back to FIG. 1, the plurality of semiconductor strips 22 has lengthwise directions parallel to each other. The neighboring semiconductor strips 22 may have a uniform pitch. A plurality of gate stacks 28 are formed over semiconductor strips 22, and may be formed to contact the sidewalls and the top surfaces of semiconductor strips 22. Each of gate stacks 28 includes a gate dielectric layer and a gate electrode (not shown).

Well regions 26 (including 26A and 26B) are formed, and portions 22B of semiconductor strips 22, which portions 22B are covered by gate stacks 28, may also be portions of well regions 26. In some embodiments, well regions 26 are n-well regions, which may have an n-type impurity concentration between about 1E14/cm³ and about 1E17/cm³, for example. Accordingly, the portions 22B of semiconductor strips 22 are of n-type. Semiconductor strips 22 include heavily doped p-type (p+) regions 22A and heavily doped n-type (n+) regions 22C, which have a p-type impurity concentration and an n-type impurity concentration, respectively, between about 1E19/cm³ and about 5E21/cm³, for example.

Semiconductor strips 22 form diodes D1 and D2. Each of diodes D1 and D2 has p-type semiconductor strip portions 22A as the anode, and n-type semiconductor strip portions 22B and 22C as the cathode. The anode and the cathode of diode D1 is on the top left part and top right part of SCR/diode-string unit 20. The anode and the cathode of diode D2 is on the bottom right part and bottom left part of SCR/diode-string unit 20. Accordingly, the p+ regions 22A and n+ regions 22C are allocated in a checker board pattern, wherein the checker board pattern have black and white patterns allocated in an alternating layout in each row and each column of a checker board.

Metal connections 30, which may be contact plugs, are formed for interconnection. Through metal connections 30, all semiconductor strips 22A in diode D1 are interconnected. All semiconductor strips 22C in diode D1 are interconnected. All semiconductor strips 22A in diode D2 are interconnected. All semiconductor strips 22C in diode D2 are interconnected. Furthermore, semiconductor strips 22C of diode D1 is connected to semiconductor strips 22A in diode D2, for example, through metal connections (such as metal lines, vias, and contact plugs) 32. Accordingly, diodes D1 and D2 form a diode string. The anode 22A of diode D1 may be connected to input/output pad 34 through metal connections 32. The cathode 22C of diode D2 may be connected to Vss node 36 through metal connections 32, which may be an electrical ground. Diodes D1 and D2 are biased in the same direction, with the cathode of each of diodes D1 and D2 closer to Vss node 36 than the respective anode of the same diode.

FIG. 2 illustrates a cross-sectional view of the structure shown in FIG. 1, wherein the cross-sectional view is obtained from the plane containing A-A in FIG. 1. In some embodiments, substrate 38 is a p-type substrate (p-sub). Accordingly, the structure in FIG. 2 form PNP bipolar transistor PNP1 and NPN bipolar transistor NPN1. Bipolar transistor PNP1 includes p+ strips 22A as the emitter, n-well region 26A as the base, and p-sub 38 as the collector. Bipolar transistor NPN1 has n+ strips 22C and n-well region 26B as the emitter, p-sub 38 as the base, and n-well region 26A as the collector. Bipolar transistors PNP1 and NPN1 in combination form an SCR, which is referred to as SCR1 hereinafter.

FIG. 3 illustrates the circuit diagram of SCR/diode-string unit 20, wherein diodes D1 and D2 and bipolar transistors PNP1 and NPN1 are illustrated. The SCR1 is triggered by the currents of diode string D1 and D2. SCR/diode-string unit 20 has a high ESD discharging ability due to the high ESD discharging ability of SCR1. On the other hand, diodes D1 and D2 are also coupled between pad 34 and Vss node 36. Accordingly, SCR/diode-string unit 20 has a high turn-on speed due to the high turn-on speed of diodes D1 and D2. The high turn-on speed is advantageous for the ESD protection in Charge Device Mode (CDM).

FIG. 4 illustrates a top view of an ESD protection device, which includes a plurality of SCR/diode-string units 20 connected in parallel. Each of SCR/diode-string units 20 is connected to pad 34 and Vss node 36. The p+ regions 22A and n+ regions 22C are allocated in a checker board pattern. These embodiments have good ESD discharging ability since the ESD discharging current is multiplied. On the other hand, since the diode string in each of SCR/diode-string units 20 is coupled between pad 34 and Vss node 36, the turn-on speed is high.

FIG. 5A illustrates SCR/diode-string combo unit 44, which includes two SCR/diode-string units 20 (including 20A and 20B having an identical structure) cascaded between input/output pad 34 and Vss node 36. The p+ regions 22A and n+ regions 22C (refer to FIG. 1) in these embodiments are also allocated in a checker board pattern. In these embodiments, metal connection 46 is formed to connect the cathode of diode D2 in SCR/diode-string unit 20A to the anode of diode D3 in SCR/diode-string unit 20B. Accordingly, SCR/diode-string unit 44 includes a diode string including four diodes D1, D2, D3, and D4 biased in the same direction. With the diode string including four diodes, the holding voltage of SCR/diode-string combo unit 44 is increased over the holding voltage of a single SCR/diode-string unit 20.

FIG. 5B illustrates SCR/diode-string combo unit 44 in accordance with alternative embodiments. These embodiments are similar to the embodiments in FIG. 5A, except that there are three diodes D1, D2, and D3 (rather than four diodes) forming the respective diode string.

In alternative embodiments, more (such as 3, 4, 5, and 6) SCR/diode-string units 20 may be cascaded to form a SCR/diode-string combo unit. As can be found from FIGS. 1 and 5, the holding voltage of the SCR/diode-string combo units in accordance with the exemplary embodiments may be adjusted by changing the number of cascaded SCR/diode-string units 20. FIG. 13 illustrates the holding voltages of the SCR/diode-string combo units as a function of the number of cascaded SCR/diode-string units 20 (FIG. 1). For example, when SCR/diode-string unit 20 has two to three diodes, as shown in FIG. 1, the holding voltage may be about 0.9 volts. The SCR/diode-string combo unit 44 as shown in FIG. 5A, which includes two SCR/diode-string units 20, and hence four diodes, serially connected, has a holding voltage of 1.8 volts. Three serially connected diodes may also have the holding voltage of 1.8 Volts. When the number of cascaded SCR/diode-string units 20 increases to 4˜5 or 5˜6, which correspond to 8˜10 or 10˜12 serially connected diodes, respectively, the holding voltage may increase to about 3.3 volts and about 5 volts, respectively.

FIG. 6 illustrates a cross-sectional view of the structure shown in FIG. 5A, wherein the cross-sectional view is obtained from the plane containing line B-B in FIG. 5A. As shown in FIG. 6, three SCRs are formed. SCR1 is the SCR formed by SCR/diode-string unit 20A. SCR1 includes n-well regions 26A and 26B, and the overlying p+ portions 22A and n+ portions 22C. SCR2 is the SCR formed by SCR/diode-string unit 20B. SCR2 includes n-well regions 26C and 26D, and the overlying p+ portions 22A and n+ portions 22C. SCR3 includes n-well regions 26A and 26D, and the overlying p+ portions 22A and n+ portions 22C. The details of each of SCR1, SCR2, and SCR3 may be found referring to the discussion of FIG. 2.

FIG. 7 illustrates a circuit diagram of SCR/diode-string combo unit 44, wherein transistors PNP1 and NPN1 form SCR1 (FIG. 6), transistors PNP2 and NPN2 form SCR2 (FIG. 6), and transistors PNP1 and NPN3 form SCR3 (FIG. 6). Diodes D1, D2, D3, and D4 are serially connected between pad 34 and Vss node 36. The SCRs SCR1, SCR2, and SCR3 are triggered by the currents of the diode string D1, D2, D3, and D4. Similarly, SCR/diode-string combo unit 44 has a high turn-on speed due to the high turn-on speed of diodes D1, D2, D3, and D4.

FIG. 8A illustrates a diode-string-free SCR unit 50 in accordance with some embodiments. Diode-string-free SCR unit 50 includes SCRs that are connected between nodes 34 and 36, and does not include any diode string that are connected directly from node 34 to node 36, with all diodes in the diode string biased in the same direction. In these embodiments, each of semiconductor strips 22 in diode-string-free SCR unit 50 is doped to a same conductivity type, with semiconductor strips 22A being of p-type, and semiconductor strips 22C being of n-type. Therefore, the portions of the same semiconductor strip 22 on the opposite sides of gate stacks 28 are of the same conductivity type, and hence do not form a diode.

In FIG. 8A, SCRs SCR1, SCR2, and SCR3 are marked, wherein each of the SCRs SCR1, SCR2, and SCR3 is formed of a p+ strip, an n+ strip, the underlying n-well regions 26, and the p-sub 38 underlying n-well regions 26. Accordingly, diode-string-free SCR unit 50 has a similar ability for conducting high ESD currents as SCR/diode-string combo units 44 (FIGS. 5 through 7).

FIG. 8B illustrates diode-string-free SCR unit 50 in accordance with alternative embodiments. These embodiments are similar to the embodiments in FIG. 8A, except that the diode-string-free SCR unit 50 in these embodiments includes a half (the left half or the right half) of the diode-string-free SCR unit 50 in FIG. 8A. Alternatively stated, the diode-string-free SCR unit 50 in FIG. 8A may be assembled by placing two of the diode-string-free SCR unit 50 in FIG. 8B together.

SCR/diode-string units 20 (FIG. 1) and SCR/diode-string combo units 44 (FIGS. 5A and 5B) have the advantageous features of high turning-on speed, and hence are capable of reducing CDM overshoot. On the other hand, diode-string-free SCR unit 50 (FIG. 8A or 8B) has more SCRs, and hence can provide better ESD protection for high-ESD-current applications. Accordingly, by combining SCR/diode-string units 20 and SCR/diode-string combo units 44 with diode-string-free SCR unit 50, the requirement of reducing CDM overshoot and conducting high ESD current may be satisfied.

FIGS. 9, 10, 11, and 12 illustrate the top views of portions of exemplary ESD protection circuits. It is appreciated that the ESD protection circuits may include more portions repeating the illustrated portions. FIGS. 9, 10, 11, and 12 have different ratios of the number of SCR/diode-string combo units 44 to the number of diode-string-free SCR units 50. The ESD protection circuit in FIG. 9 includes SCR/diode-string combo units 44, and does not include any diode-string-free SCR unit 50. Accordingly the ratio is 1:0. The ESD protection circuit in FIG. 10 includes one SCR/diode-string combo unit 44 corresponding to each diode-string-free SCR unit 50. Accordingly, the ratio is 1:1. The ESD protection circuit in FIG. 11 includes one SCR/diode-string combo units 44 corresponding to every two diode-string-free SCR units 50. Accordingly, the ratio is 1:2. The ESD protection circuit in FIG. 12 includes one SCR/diode-string combo unit 44 corresponding to every three diode-string-free SCR units 50. Accordingly, the ratio is 1:3.

The embodiments of the present disclosure have several advantageous features. The formation of the ESD protection circuit is fully compatible with the manufacturing process for forming Fin Field-Effect Transistors (FinFETs), regardless of the spacing between the semiconductor strips. Furthermore, the trigger voltage and the holding voltage of the ESD protection circuit may be adjusted by changing the number of cascaded diodes in the diode string. Furthermore, the requirements for reducing CDM overshoot and conducting high ESD current may be balanced through adjusting the ratio of the number of SCR/diode-string combo units to the number of diode-string-free SCR units.

In accordance with some embodiments, an ESD protection circuit includes a plurality of groups of p-type heavily doped semiconductor strips (p+ strips) and a plurality of groups of n-type heavily doped semiconductor strips (n+ strips) forming an array having a plurality of rows and columns. In each of the rows and the columns, the plurality of groups of p+ strips and the plurality of groups of n+ strips are allocated in an alternating layout. The ESD protection circuit further includes a plurality of gate stacks, each including a first edge aligned to an edge of a group in the plurality of groups of p+ strips, and a second edge aligned to an edge of a group in the plurality of groups of n+ strips. The ESD protection circuit further includes a conductor electrically connecting a first one of the plurality of groups of p+ strips to a second one of the plurality of groups of n+ strips, wherein the first one and the second one are in a same column.

In accordance with other embodiments, an ESD protection circuit includes a semiconductor substrate of a first conductivity type, and a first well region and a second well region of a second conductivity type opposite to the first conductivity type. The first well region and the second well region are separated from each other by a portion of the semiconductor substrate. A first semiconductor strip extends in a row direction and overlaps and contacts the first well region. The first semiconductor strip includes a first heavily doped portion of the first conductivity type, a second heavily doped portion of the second conductivity type, and a third portion of the second conductivity type connecting the first portion to the second portion. A first gate stack overlaps the third portion of the first semiconductor strip. A second semiconductor strip extends in the row direction and overlaps and contacts the second well region. The second semiconductor strip includes a fourth heavily doped portion of the first conductivity type, a fifth heavily doped portion of the second conductivity type, and a sixth portion of the second conductivity type connecting the fourth portion to the fifth portion. The first and the fifth portions are in a same first column. The second and the fourth portions are in a same second column. A second gate stack overlaps the sixth portion of the second semiconductor strip. A first conductor electrically connects the second portion to the fourth portion. The first and the second gate stacks and the first and the second semiconductor strips are comprised in an SCR/diode-string unit.

In accordance with yet other embodiments, an ESD protection circuit includes a p-type semiconductor substrate, and a diode string including a first, a second, a third, and a fourth diode. The first, the second, the third, and the fourth diodes are aligned sequentially in a column. Each of the first, the second, the third, and the fourth diodes includes a p+ semiconductor strip as an anode, and an n-type semiconductor strip and a n+ semiconductor strip as a cathode. The anodes and the cathodes of the first, the second, the third, and the fourth diodes form an array. In each of rows and columns of the array, the anodes and the cathodes are allocated in an alternating layout. The ESD protection circuit further includes a gate electrode overlaps the n-type semiconductor strip, and four n-well regions, each overlapped by and in contact with one of the first, the second, the third, and the fourth diodes. The four n-well regions are separated from each other by portions of the p-type semiconductor substrate.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. An Electro-Static Discharge (ESD) protection circuit comprising: a plurality of groups of p-type heavily doped semiconductor strips (p+ strips); a plurality of groups of n-type heavily doped semiconductor strips (n+ strips), wherein the plurality of groups of p+ strips and the plurality of groups of n+ strips form an array having a plurality of rows and columns, and wherein in each of the rows and the columns, the plurality of groups of p+ strips and the plurality of groups of n+ strips are allocated in an alternating layout; a plurality of gate stacks, each comprising: a first edge aligned to an edge of a group in the plurality of groups of p+ strips; and a second edge aligned to an edge of a group in the plurality of groups of n+ strips; and a conductor electrically connecting a first one of the plurality of groups of p+ strips to a second one of the plurality of groups of n+ strips, wherein the first one and the second one are in a same semiconductor strip column.
 2. The ESD protection circuit of claim 1 further comprising: an input/output pad; a Vss node; and a plurality of conductors comprising the conductor, wherein each of the plurality of conductors electrically connects one of the plurality of groups of p+ strips in a semiconductor strip column to one of the plurality of groups of n+ strips in the semiconductor strip column.
 3. The ESD protection circuit of claim 2 comprising a plurality of diodes forming a diode string, wherein each of the plurality of diodes comprises one of the group in the plurality of groups of p+ strips as an anode and one of the group in the plurality of groups of n+ strips as a cathode, and wherein the plurality of diodes are serially connected between the input/output pad and the Vss node.
 4. The ESD protection circuit of claim 3, wherein the cathode of each of the diodes in the diode string is between the Vss node and the anode of the each of the diodes in the diode string.
 5. The ESD protection circuit of claim 3, wherein the plurality of diodes comprises two diodes.
 6. The ESD protection circuit of claim 5, wherein the plurality of diodes comprises fourth diodes.
 7. The ESD protection circuit of claim 1, wherein the array comprises at least four columns and at least two rows.
 8. An Electro Static Discharge (ESD) protection circuit comprising: a semiconductor substrate of a first conductivity type; a first well region and a second well region of a second conductivity type opposite to the first conductivity type, wherein the first well region and the second well region are separated from each other by a portion of the semiconductor substrate; a first semiconductor strip extending in a row direction and overlapping and contacting the first well region, wherein the first semiconductor strip comprises: a first heavily doped portion of the first conductivity type; a second heavily doped portion of the second conductivity type; and a third portion of the second conductivity type connecting the first portion to the second portion; a first gate stack overlapping the third portion of the first semiconductor strip; and a second semiconductor strip extending in the row direction and overlapping and contacting the second well region, wherein the second semiconductor strip comprises: a fourth heavily doped portion of the first conductivity type; a fifth heavily doped portion of the second conductivity type; and a sixth portion of the second conductivity type connecting the fourth portion to the fifth portion, wherein the first and the fifth portions are in a same first column, and wherein the second and the fourth portions are in a same second column; a second gate stack overlapping the sixth portion of the second semiconductor strip; and a first conductor electrically connecting the second portion to the fourth portion, wherein the first and the second gate stacks and the first and the second semiconductor strips are comprised in a first Silicon-Controlled Rectifier (SCR)/diode-string unit.
 9. The ESD protection circuit of claim 8 further comprising: an input/output pad electrically connected to the first heavily doped portion; and a Vss node electrically connected to the fifth heavily doped portion.
 10. The ESD protection circuit of claim 8 further comprising: a second SCR/diode-string unit having a structure identical to the first SCR/diode-string; and a second conductor electrically connecting the fifth portion of the first SCR/diode-string unit to the first portion of the second SCR/diode-string unit.
 11. The ESD protection circuit of claim 8 further comprising: a second SCR/diode-string unit having a structure identical to, and connected to in parallel with, the first SCR/diode-string, wherein the first semiconductor strip of the first SCR/diode-string unit and the first semiconductor strip of the second SCR/diode-string unit are portions of a same straight and continuous semiconductor strip.
 12. The ESD protection circuit of claim 8, wherein the first conductivity type is p-type, and the second conductivity type is n-type.
 13. The ESD protection circuit of claim 8: wherein the first semiconductor strip further comprises: a seventh heavily doped portion of the first conductivity type; an eighth heavily doped portion of the first conductivity type; and a ninth portion of the first conductivity type connecting the seventh portion to the eighth portion; wherein the second semiconductor strip further comprises: a tenth heavily doped portion of the second conductivity type; an eleventh heavily doped portion of the second conductivity type; and a twelfth portion of the second conductivity type connecting the tenth portion to the eleventh portion, and wherein the ESD protection circuit further comprises: a third gate stack over the ninth portion of the first semiconductor strip; and a fourth gate stack over the twelfth portion of the second semiconductor strip, wherein the seventh and the twelfth portions are in a same third column, and wherein the eighth and the tenth portions are in a same fourth column.
 14. The ESD protection circuit of claim 8 further comprising shallow trench isolation regions, and wherein the first and the second semiconductor strips comprise first portions extending between the shallow trench isolation regions, and second portions over top surfaces of the shallow trench isolation regions.
 15. An Electro-Static Discharge (ESD) protection circuit comprising: a p-type semiconductor substrate; a diode string comprising a first, a second, a third, and a fourth diode, wherein the first, the second, the third, and the fourth diodes are aligned sequentially in a column, and wherein each of the first, the second, the third, and the fourth diodes comprises: a p+ semiconductor strip as an anode; an n-type semiconductor strip and a n+ semiconductor strip as a cathode, wherein the anodes and the cathodes of the first, the second, the third, and the fourth diodes form an array, and wherein in each of rows and columns of the array, the anodes and the cathodes are allocated in an alternating layout; and a gate electrode overlapping the n-type semiconductor strip; and four n-well regions, each overlapped by and in contact with one of the first, the second, the third, and the fourth diodes, wherein the four n-well regions are separated from each other by portions of the p-type semiconductor substrate.
 16. The ESD protection circuit of claim 15 further comprising an input/output pad and a Vss node, wherein the first, the second, the third, and the fourth diodes are serially coupled between the input/output pad and the Vss node.
 17. The ESD protection circuit of claim 16 further comprising an additional diode string having an identical structure as the diode string, wherein the additional diode string is connected in parallel with the diode string.
 18. The ESD protection circuit of claim 15, wherein the first, the second, the third, and the fourth diodes are biased in a same direction.
 19. The ESD protection circuit of claim 15, wherein the diode string comprises eight serially connected diodes.
 20. The ESD protection circuit of claim 15 further comprising: a first conductor connecting the cathode of the first diode to the anode of the second diode; a second conductor connecting the cathode of the second diode to the anode of the third diode; and a third conductor connecting the cathode of the third diode to the anode of the fourth diode. 